The ever-increasing number of sensor nodes on and around the human body is rapidly transforming the way people interact with their surroundings. The Internet of Things (IoT) has gained considerable traction over the past decade and continued research is forecasted to expand the network to reach from 24 to 50 billion body-worn devices. A perennial obstacle to realizing the deployment of body-worn devices is power. Many body-worn devices may require long operational lifetimes, ideally without the need to replace a battery. Moreover, depending on the application, many sensors may have size and weight constraints, rendering current battery technology inappropriate. Energy harvesting has been seen by many researchers as an emerging solution to meet the power demands of body-worn devices. Of the many ambient sources available, including electromagnetic radiation, thermal gradients, and mechanical motion, solar harvesting is a common choice due to a good balance of power density and packaging flexibility (e.g., enabling form-fitting when recharging wearables).
However, for applications not exposed to sunlight, alternative scavenging sources may be utilized. For example, it may be possible to extract energy from naturally occurring phenomena such as low-level vibrations present in households and office environments and fluidic motion found in evaporation. Unfortunately, small (e.g., centimeter scale or smaller) body-worn devices cannot efficiently make use of ambient environment vibration or fluid flow.
The human body has the potential to be a very valuable source of power for body-worn devices (e.g., biosensors), storing approximately 384 megajoules (MJ) of potential energy in fat for an average sized person. Although most of this chemical energy is not directly harvestable today, a small fraction of the available power can be indirectly scavenged from human temperature gradients and everyday motion. Although the heels of walking shoes are some of the most profitable locations to scavenge energy, it can be difficult to deliver this power to a useful device located somewhere on the body or in the environment. Human finger motion, on the other hand, is a natural means of communication which could generate energy through piezoelectric, triboelectric, and electrostatic transduction.